MPG Vs. Wheel/Tyre Size

almillar

OP has normalised the situation - the TYRE is the same width and circumfrence, so the same amount of rubber is in contact with the road, with the same amount of friction. Assume also that it's the same brand etc of tyre, so grip and air resistance will be the same too. The 15" wheel, like for like, should weigh less than the 17, so it wins.
If we're being geeks about it (and we are!), the 15 inch wheel with the higher profile tyres will provide more protection for the suspension components (and wheels) than the lower profile 17s, so you'll save money in component replacement in the VERY long run.

Lum

That's a very good point on the suspension. You're also less likely to get damage to the wheel itself from curbing* which may improve the resale value of your car.

Basically the only reasons to go for larger wheels is if you desire the affects on the handling, what your car to look a bit sportier/blingier or you want to fit bigger brakes that will not physically fit under the smaller size wheel**.

The fact that you are asking about the tiny MPG changes from the different wheel size means you probably don't fall into any of the above categories and should go with the 15s.

* Somebody will be along soon to tell you how they are a perfect driver and have never curbed a wheel and if you are taking this into consideration then you shouldn't be on the road.
** This is the reason why I run 16s on my Nissan instead of the stock 15s.

mikey72

almillar wrote: »

OP has normalised the situation - the TYRE is the same width and circumfrence, so the same amount of rubber is in contact with the road, with the same amount of friction. Assume also that it's the same brand etc of tyre, so grip and air resistance will be the same too. The 15" wheel, like for like, should weigh less than the 17, so it wins.
If we're being geeks about it (and we are!), the 15 inch wheel with the higher profile tyres will provide more protection for the suspension components (and wheels) than the lower profile 17s, so you'll save money in component replacement in the VERY long run.

Like for like, the 17" will be better.

The main rolling resistance of a tyre is caused by the deformation of the sidewalls as they roll, so the tyre is never truely round.
So energy is needed to push the tyre round, the flex on the sidewall creates the flat spot at the bottom, and energy is needed to keep moving the deforming sidewall as the tyre changes position.
(Hence why cars with low pressure in the tyres use more petrol than those inflated correctly, or a few psi over, and why a car with flat tyres is harder to push)

The power profile tyres have less sidewall to deform, and also a firmer sidewall to make up for the lack of height, so are easier to roll.

Energy savers have a harder sidewall for the same reason.